The physical downsizing of microprocessor based technologies has led to portable personal computers, pocket secretaries, wireless phones and pagers. All of these devices, and also other devices such as clocks, watches, calculators, etc., have the common need for a low power consumption data display screen to extend the useful working time between battery replacements or battery charges. The common Liquid Crystal Display (LCD) is often used as the display for such devices. LCDs can be classified based upon the source of illumination. Reflective displays are illuminated by ambient light that enters the display from the front. In applications where the intensity of ambient light is insufficient for viewing, supplemental lighting, such as a backlight assembly, is used to illuminate the display. Some electronic displays have been designed to use ambient light when available and backlighting only when necessary. This dual function of reflection and transmission leads to the designation, “transflective”.
A liquid crystal display (LCD) illustrates another example of the use of polarized light. FIGS. 1A and 1B schematically illustrate one example of a simple TN (twisted nematic) LCD device with E-mode transmission and normally white (NW) operation using a backlight. It will be understood that there are a variety of other LCD types and other modes of operation, as well as displays that use ambient light or a combination of a backlight and ambient light. The inventions discussed herein can be readily applied to these display types and modes of operation.
The LCD 50 of FIGS. 1A and 1B includes a liquid crystal (LC) cell 52, a polarizer 54, an analyzer 56, and a backlight 58. The arrows 55, 57 on the polarizer 54 and analyzer 56, respectively, indicate the polarization of light that is transmitted through that component. Arrows 51, 53 indicate the plane of polarization of linearly polarized light, respectively entering and exiting the LC cell 52. Additionally, the plane of the LC cell 52 containing arrows 51, 53 generally includes transparent electrodes. Light from the backlight 58 is linearly polarized by the polarizer 54. In the embodiment illustrated in FIG. 1A, in the absence of an electric potential applied across the LC cell, the director substantially lies in the plane of the display twisting uniformly through 90° along its depth. The polarized light is transmitted through the LC cell 52 where the polarization ideally rotates by 90°, with the director of the liquid crystals indicated by the arrows 51, 53. This light can then be transmitted through the analyzer 56.
An electric potential can be applied at electrodes (not shown) proximate to opposing ends of the LC cell 52, setting up an electric field within the LC cell. In the case where the LC material has a positive dielectric anisotropy, the director substantially aligns in the direction of the electric field lines, provided sufficient potential is applied across the electrodes. The director at the center of the cell is oriented perpendicular to the plane of the display in this case. The linearly polarized light entering the cell is no longer rotated through the 90° required for transmission through the analyzer. In the embodiment illustrated in FIG. 1B, the plane of polarization of the polarized light as it exits LC cell 52 (designated by arrow 53′) is unchanged from its original orientation (designated by arrow 51). Hence, the light exiting the LC cell 52 is not transmitted through the analyzer 56, because the light exiting the LC cell has the wrong polarization. One method of obtaining a gray scale includes only applying sufficient electric potential to partially orient the director of the liquid crystals between the two illustrated configurations. In addition, it will be recognized that a color cell can be formed by, for example, using color filters.
Typically, the polarizer 54 and analyzer 56 are constructed using absorbing sheet polarizers because these polarizers have good extinction of light having the unwanted polarization. This, however, results in substantial loss of light because the backlight generally emits unpolarized light. Light of the unwanted polarization is absorbed by the polarizer. As an alternative configuration (illustrated in FIG. 1C), a reflective polarizer 60 is placed between the polarizer 54 and the backlight 58. The reflective polarizer reflects light with the unwanted polarization back towards the backlight. The reflected light can be recycled using a reflector 62 behind the backlight where a substantial portion of the reflected light can be reused.